1. Technical Field
The present invention relates to a vibration type tilting device, which is positioned on a light path and which periodically performs tilting to reflect light emitted from a light source, and to a damping adjustment method therefor.
2. Description of the Related Art
A vibration type tilting device, often used in projection TV's, etc., is positioned on a light path and periodically tilts the light projected to a screen in minute angles to create a visual afterimage effect and provide a more natural picture.
FIG. 1 is a perspective cross-sectional view of a conventional vibration type tilting device, and FIG. 2 is a cross-sectional view of the conventional vibration type tilting device illustrated in FIG. 1. The vibration type tilting device depicted in FIGS. 1 and 2 includes a mirror 1 which reflects light emitted from a light source (not shown), a mirror holder 2 supporting the mirror 1, coils 3 attached on the reverse side of the mirror holder 2 in bilateral symmetry, cores 5 inserted inside the coils 3, and magnets 7 and yokes 9 positioned at the upper portions of the cores 5. Also, viscous fluid 6 is inserted between the coil 3 and the yoke 9 and between the coil 3 and the core 5.
When an electric current is supplied to the coils, an electric field is formed, where the interaction between this electric field and the magnetic field generated by the magnets 7 generates a vibrational force which tilts the mirror holder 2. The viscous fluid 6 performs damping on the vibrating coils 3 to improve the vibration performance of the mirror holder 2, e.g. by reducing rising time or overshoot.
FIG. 3 is a graph representing the temperatures of the mirror holder 2 and coils 3 during operation of a conventional vibration type tilting device.
As illustrated in FIG. 3, in a conventional vibration type tilting device, the temperature in the vicinity of the coils 3 rises to a high level of about 125° C., due to the electric current supplied to the coils 3. In particular, as the resistance of the coils 3 increases further with prolonged use of the tilting device, the concentration of heat in the vicinity of the coils 3 is further exacerbated. This rise in temperature of the coils 3, as illustrated in FIG. 2, increases the temperature of the viscous fluid 6 in the vicinity of the coils 3 and lowers the viscosity. It is noted, however, that the parts other than the coils 3 are close to normal temperature (20° C.).
FIG. 4(a) is a graph representing the vibration characteristics of a tilting device at normal temperature, and FIG. 4(b) is a graph representing the vibration characteristics of a tilting device kept for 1 hour at 80° C. In the graphs, the vertical axis represents the displacement of the mirror holder 2. As illustrated in FIG. 4(a), a conventional vibration type tilting device shows a 5.2% overshoot at room temperature, but shows a 17.3% overshoot when the temperature is increased. This increase in overshoot following a rise in temperature is because the higher the temperature of a fluid, the lower its viscosity.
Thus, in a conventional vibration type tilting device, the temperature of the viscous fluid 6 is increased with prolonged use, due to the rise in temperature of the coils 3. This causes a decrease in the viscosity of the viscous fluid, whereby overshoot is increased of the mirror holder 2 and residual vibration occurs. Also, since the amount of viscous fluid inserted is very little so that it is difficult to inject the precise amount, the damping forces may be too great, or conversely, too small.